Air pollution is a serious problem especially in large cities. In the U.S. the Environmental Protection Agency has the primary responsibility for carrying out the requirements of the Clean Air Act, which specifies that air-quality standards shall be established for hazardous substances. There are also state laws and international Protocols that set standards.
Some air pollutants are formed through the action of sunlight on previously emitted reactive materials (called precursors). For example, ozone, a pollutant in smog, is produced by the interaction of hydrocarbons and nitrogen oxides under the influence of sunlight. Although many types of combustion contribute to this problem, trucks and buses have been identified as a significant source of both oxides of nitrogen (NO.sub.x) and particulate matter (PM). Pollution from internal combustion engines has been significantly reduced by burning the fuel as completely as possible, by recirculating fumes and by using catalytic converters. However, standards are constantly being changed in an attempt to lower exhaust emissions. Current standards propose NO.sub.x emissions limits of between 1.5 and 2 grams per brake horse power per hour (g/bhp-hr). The state of California has adopted an Ultra Low Emission Vehicle (hereinafter ULEV) regulation, which will become effective in 1998, for medium-duty vehicles that limits NO.sub.x plus hydrocarbons at 2.5 g/bhp-hr and caps particulates at 0.05 g/bhp-hr. In addition this California regulation restricts the emissions of formaldehyde (HCHO) to 0.025 g/bhp-hr and Carbon Monoxide (CO) emissions to 7.2 gbhp-hr. Meeting such standards will be difficult for spark ignited (SI) engines and even more difficult.
Trying to meet such standards alternative fuels such as Methanol and Ethanol have been tried. Dimethyl ether, CH.sub.3 -O-CH.sub.3 hereinafter DME, is currently used as a propellant for spray cans. DME was adopted for this use as a replacement for chlorofluorcarbons. DME has been used in experiments, as an ignition enhancer, for Methanol-fueled Diesel Engines. However, even when the ratio of DME to Diesel Fuel is as nigh as 60%, satisfactory operation was not obtained. Recently, a limited test was conducted using pure DME as an alternative fuel in a single cylinder, four stroke, direct injection Diesel Engine. This test yielded very promising combustion, performance and emissions results. Although the fuel injection system used in this test was designed for standard diesel fuel, when using DME the thermal efficiency of the engine was equivalent to when diesel fuel is used. Furthermore, as compared to standard diesel fuel the NO.sub.x, were low and the smoke emissions were extremely low. Reference is made to a soon to be published paper by S. C. Sorenson entitled, Performance and Emissions of a 0.273 Liter Direct Injection Diesel Engine with a New Alternative Fuel, in which this test are discussed.
The use of DME as an alternative fuel does have obstacles that must be overcome. DME is a gas at ambient temperature and pressure and thus the fuel storage and delivery system must be pressurized to maintain the DME in a liquid state. DME must be pressurized to about five bar to keep it in a liquid state under ambient conditions. At the elevated temperatures present on an internal combustion engine higher pressures (12-30 bar) are required to maintain DME in a liquid state.
The energy density of DME, although higher than the alternative fuels Methanol (CH.sub.3 OH) and Ethanol (CH.sub.3 -CH.sub.2 OH) it is much lower than conventional Diesel Fuel. As a result to obtain the same power from an engine fueled by DME obtained when fueling with Diesel Fuel the volume of DME must be increased by a factor of about 1.8. To accommodate this increased volume the fuel injector must have a larger orifice opening. A single hole pintle type nozzle, rather than a multi hole nozzle, has been found to function well to provide this increased fuel flow.
A fuel's Cetane number, which is a measure of the fuel's ability to auto-ignite, has an important influence on diesel combustion and is a meaningful indicator of a fuel's value for diesel engines. Fuels with a high Cetane number will ignite quicker and thus will have a short ignition delay. This lowers premixed burning of the fuel, which in turn lowers NO.sub.x and noise emissions. DME has a higher Cetane number than Diesel Fuel and thus it will ignite quicker and will have a relatively short ignition delay. By throttling the amount of fuel injected during the initial portion of the injection cycle the quantity of fuel in the combustion chamber when ignition occurs has been diminished which significantly lowers NO.sub.x and noise emissions. The mechanism for throttling the fuel injected during the initial portion of the injection cycle should be time dependent such that it can be coordinated with ignition delay that is also time dependent.
Also, the vapor pressure of DME is higher than most other fuels. At 38.degree. Centigrade, the vapor pressure of DME is 8 bar as compared to 0.0069 bar and 0.35 bar respectively for Diesel fuel and Methanol. Thus DME will boil at a lower atmosphere pressure than Diesel fuel or Methanol. The system must be pressurized to prevent the fuel from flashing to vapor in the engine's fuel manifolds or fuel injection system.
The viscosity of DME is estimated to be about 5% to 10% of diesel fuel. This relatively low viscosity of DME portends fuel leakage in a system designed for fuels having higher viscosities. Thus, standard fuel storage and delivery systems will not be suitable for DME.
Test results, such as those described in the above referred to Sorenson paper, are obtained in carefully controlled and monitored operating environments and conditions. It is often difficult to duplicate such test results outside the laboratory. As a result further developments are required to obtain the same results in a production situation where many and changing conditions are experienced.
Internal combustion engines and especially Diesel engines represent large capital investments and have long useful lives. The current process for producing DME would result in a price that would render it unacceptable as an alternative fuel. A new less costly manufacturing method has been developed to produce "raw DME" which is a form of DME that includes small amounts of water and Methanol. Large capital investments would be required to build the necessary facilities to produce raw DME at volumes that would meet its demand as an alternative fuel. Even greater capital investments would be required to provide the necessary refueling system. Large capital investments of this magnitude are unlikely to be made if the alternative fuel can only be used in newly produced special designed engines. Thus, a very important consideration for an alternative fuel is whether economic field conversions can be made to existing engines to enable them to use the alternative fuel.
For these reasons, there is a need for a fuel storage and delivery system that will enable internal combustion engines to be powered with DME fuel in a broad range of environmental conditions. The new and improved fuel storage and delivery system must also permit existing internal combustion engines to be economically converted in the field to be fueled by DME.